The three primary colors, red, green, and blue, are provided to an organic light-emitting diode (OLED) in order to form white light. Along the development of the OLED, efficiency has advanced significantly from the fluorescence system of the first generation and the phosphorescence system of the second generation to the thermally activated delayed fluorescence (TADF) system of the third generation. Both green and red OLEDs have satisfactory efficiency and lifetime, but blue OLED still exhibits lower efficiency and much lower lifetime.
The major reason is due to the exciton-polaron annihilation of blue light device. For example, the triplet exciton energy of blue light is approximately 2.8 eV (T1), and thus its exciton lifetime is long (˜μs) and the exciton may interact with a polaron resulting in energy transferring to the polaron (D0). The polaron has already a certain level of energy (˜3.3 eV), so a hot polaron (Dn*) may be formed with high energy (>6 eV). Such a hot polaron may break a bonding in an organic material (e.g., the bonding energy of C—N is 3.04 eV merely and thus the bonding is likely to be broken), resulting in short lifetime of the blue phosphorescent OLED.
Similar problems are also recognized in the blue TADF device. As an exciton lifetime is long (˜1-10 μs), a reaction of hot exciton-polaron annihilation is inevitable.
Therefore, a behavior to considerably reduce the exciton-polaron annihilation is required, so as to create a blue light OLED and a white OLED with high efficiency and long lifetime.